Although the demand for lithium-ion batteries has been increasing in various fields such as mobile phones, automobiles, and storage batteries, because the ore source of lithium to become the negative electrode material thereof is ubiquitous worldwide, there is a growing concern about the global supply in recent years. On the other hand, sodium chloride which serves as a raw material of sodium is contained abundantly in seawater and bedded salt and is distributed globally. Accordingly, development of a sodium ion secondary battery using sodium in place of lithium has been desired.
Although sodium-sulfur (NaS) batteries have been put into practical use as the battery using sodium as a raw material, NaS batteries require a temperature of 1300° C. or higher to drive, and thus are limited to the specific applications of power storage and have not been put into practical use for general applications. In addition, although molten sodium is used as a negative electrode active material and negative electrodes are configured using a mesh made of SUS, steel wool, or the like as a current collector in the NaS batteries, because sodium metal is used in the form of liquid with high activity; there is a disadvantage in that sodium metal easily flows into the positive electrode chamber side to cause short circuit in those cases where the solid electrolyte breaks down.
In sodium secondary batteries, in order not to make sodium move to positive electrode side, it is required to adhere sodium to a current collector to prevent the break down of electrical connection therewith, thereby suppressing the flow of sodium. Accordingly, for example, those that are joined by adhering sodium metal or a sodium compound onto the surface of a current collector in layers so as to provide electrical conductivity have been used as the negative electrodes of sodium secondary batteries.
As a method of coating the surface of a support with sodium metal, for example, a method of producing a sodium metal-coated body has been known, which is characterized in that sodium metal is dissolved in liquid ammonia and the resulting solution is brought into contact with a support having good air permeability and a large surface area, followed by the volatilization of ammonia (for example, see Patent Document 1).
In addition, in the methods of producing sodium secondary batteries, a method of producing a negative electrode has been known, in which an appropriate amount of N-methyl pyrrolidone is added and mixed with a mixture prepared by mixing a negative electrode active material and polyvinylidene fluoride at a ratio of 95:5 to obtain a coating material-like slurry; a masking tape is adhered onto a part of copper foil with a thickness of 10 μm; the aforementioned slurry is coated onto the surface using a doctor blade, followed by drying to form a coating film; then, a similar coating film is formed on the opposite surface side, followed by application of a roll press, thereby preparing an electrode having a width of about 55 mm, length of about 330 mm, and thickness of about 230 μm to form a negative electrode; and the metallic sodium cut out into a width of 5 mm, length of 20 mm, and thickness of 200 μm is further pressure bonded onto one end of that negative electrode (for example, see Patent Document 2). As an improved method of this method for producing a negative electrode, a method has been known, in which a negative electrode is immersed in a solution prepared by dissolving sodium metal in liquid ammonia which is placed in a vessel cooled to about −40° C., before the negative electrode is pressure bonded with sodium metal, and is then taken out and charged into a vacuum chamber at room temperature to carry out the removal of ammonia, thereby producing a negative electrode predoped with sodium ions (for example, see Patent Document 3).